In core_output_filter(), reset nbytes to zero after calling

writev_it_all() in case we have another brigade to process.
Otherwise, nbytes is bogus for the second brigade and
writev_it_all() doesn't know when to quit (it loops).

In writev_it_all(), when compensating for bytes already sent
don't go beyond the number of iovs we were passed on input.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86391 13f79535-47bb-0310-9956-ffa450edef68